This invention relates to an electron beam control apparatus for use in a transmission electron microscope, and more particularly to an electron beam control apparatus for use in a transmission electron microscope that can project an electron beam probe onto an unvaried or fixed position on a sample so that the electron beam probe is prevented from slipping away, or moving, on an observation screen when the diameter of the electron beam probe is changed. The diameter of the electron beam probe may be changed for the purpose of selecting the brightness of an electron microscope or selecting the region for X-ray analysis.
Generally, electron microscopes require the electron optical condition to be optimized for each of the observation modes e.g. a high magnification mode, a low magnification mode, a bright/dark visual field mode, an ultra-high dispersion diffraction mode that means the state where a camera length is enlarged to a length of 10 m to 100 m using an imaging system. To this end, prior art electron microscopes individually control the current to be supplied to astigmatism correction means and electron beam deflection means for each of the observation modes by storing the current level for each mode (see U.S. Pat. No. 4,451,737). However, the prior art electron microscope allowes, for the same observation mode, the slipping-away or fluctuation of the electron beam probe on the observation screen due to the change in excitation current in the projection lens system. This is because the astigmatism correction means and the electron beam diffraction means are not controlled for each of the diameters of the electron beam probes used. The prior art does not take into consideration the slipping-away or fluctuation of the electron beam probe.
Thus, the prior art has an inconvenience: when the diameter of the electron beam probe to be projected to a sample is changed by varying the amount of excitation in the projection lens system, the electron beam probe moves or fluctuates on the sample due to very minute axis variations in the projection lens system, and thus the electron beam probe is projected on a different position on the sample from before.
The prior art has an additional inconvenience: since an imaging lens system has enlarged the electron beam transmitted through the sample, the movement of the projected electron beam probe is correspondingly increased, and thus the resulting image or visual field slips from the fluorescent plate and so can not be observed. Whenever this occurs, an operator must undertake the troublesome operation of readjusting the electron beam deflection device and the astigmatism correction device.